The overall objective of the proposed research is to investigate the perception and development of the phonetic environment in the very young infant. Within this objective specific studies have been proposed to investigate some of the phonetic contrasts (vowels and consonants), which the infant may be capable of distinguishing. A second feature of these studies is that they will provide information regarding the manner in which these phonetic distinctions are perceived, that is, in a linguistic or auditory mode. A third purpose is to examine the role of early receptive (perceptual) experience with language on infant speech perception. Finally, additional studies have been designed, using adult listeners, which will investigate the existence of linguistic feature detectors and their role in speech perception. All of the infant studies will use synthetic speech patterns and a procedure in which the presentation of an auditory stimulus in contingent upon the infant's production of high-amplitude sucking responses. When the infant's rate of sucking indicates a diminution of the reinforcing properties of the first auditory stimulus, a second auditory stimulus is introduced. Recovery of the sucking rate correlated with a shift in stimulation (given appropriate control groups) may be taken as inferential evidence for discrimination of the sounds. By proper comparisons of the infant's ability to discriminate two sounds from either the same or different adult phonetic categories, evidence will be available related to the nature of infant speech perception. Related information will be obtained from studies that compare the perception of linguistically relevant acoustic information in speech and nonspeech contexts. The role of experience will be assessed in a study that compares the infant's perception of phonetic distinctions that are and are not phonemic in English. The adult studies will use a selective adaptation procedure. The existence of feature detectors is shown by the systematic alterations in the identification and discrimination functions after adaptation.